Image capture indoors under low ambient lighting conditions typically requires the use of a strobe or flash device to provide supplemental illumination during image capture. Light provided from a strobe or flash device typically has a light characteristic that approximates the characteristics of natural light.
Light characteristics may be described by the color temperature of emitted light. Color temperature corresponds to the temperature of a black-body radiator when light of a certain color characteristic, or hue, is emitted from the black-body radiator. For example, ambient outdoor light on a sunny day at noon exhibits a color temperature of approximately 5,500-6,500 degrees Kelvin (° K.). A burning wax candle exhibits a color temperature of 1,850° K., corresponding to a yellowish hue.
The color temperature of strobes or flash devices are in the 5,000° K. to 6,000° K. range. Incandescent lighting, often used indoors to provide ambient lighting, exhibits a color temperature in the 2,500° K.-2,700° K. range. Incandescent light provides a yellow hue to objects. Fluorescent lighting is another often used indoor lighting source. Fluorescent lighting exhibits a color temperature in the 3,400° K.-4,200° K. range for a “cool white” fluorescent lamp, resulting in a greenish light. Full spectrum fluorescent lamps may exhibit color temperatures to 5,850° K., resulting in more natural light.
When an image is captured indoors in situations wherein ambient light is provided by fluorescent lighting or incandescent lighting, in the absence of supplemental illumination from a strobe or flash device, the captured image will show the objects as having a greenish hue or a yellowish hue, respectively. Accordingly, it is desirable to provide color correction to captured images so that the objects in the captured image appear as they would under natural sunlight conditions.
Filters may be attached to the strobe lens to provide color correction. However, the photographer must have an advanced understanding of lighting theory and image capture techniques to be able to select the appropriate filter for a particular lighting situation.
Special types of film have been developed for film-based image capture devices. Accordingly, the photographer must insert the specialized type of film that is configured to provide color correction for a particular lighting situation. Although such specialized films are easily used by a novice photographer, once loaded, the entire roll of specialized film must be used before lighting conditions change (such that another film type is required). Or if the roll is not used, the roll must be rewound such that the unused film cannot be later used.
Some digital cameras employ color correction algorithms. Such algorithms interpret sensed ambient light conditions at the time of image capture. As the electronic data of the captured image is processed and saved, the captured image data is adjusted to provide appropriate color correction. The correction may be based upon the presence or absence of supplemental illumination provided by a strobe or flash device. Color correction provided by digital cameras is very convenient in that the correction may be automated to a high degree. Accordingly, even a novice photographer can take reasonable quality, color-corrected captured images.
However, none of the above-described techniques of capturing an image using supplemental illumination from a strobe or flash device addresses the effect of the limited distance that the supplemental illumination from a strobe or flash device is cast. Furthermore, with some types of strobes or flash devices, the range of the supplemental illumination is limited in that a lens and/or a reflector is used to project the supplemental illumination in a particular direction.
Accordingly, there may be some regions of the captured image that do not receive any, or relatively little, of the supplemental illumination from a strobe or flash device because these regions are too far away to be lit, or because these regions are not within the projection range of light provided by the strobe or flash device. Such regions will exhibit the hue of the ambient lighting conditions. For example, objects in the background of a captured image may exhibit a yellowish hue when the ambient light on the background region is provided by incandescent lamps. It would be desirable to provide color correction to these regions so that all portions of the captured image exhibit a hue that more closely represents natural lighting conditions.